An exhaust gas purifying catalyst has been being widely used in order to remove harmful substances such as a hydrocarbon (HC)-based compound, carbon monoxide (CO) and nitrogen oxide (NOx), which are contained in exhaust gas discharged from an internal combustion engine.
The exhaust gas purifying catalyst is a catalyst composed of one catalyst layer or by stacking two or more catalyst layers on one another. Each of the catalyst layers is formed in such a manner that powder in which particles of noble metal such as platinum (Pt) are supported on a support containing, as a main component, a porous inorganic material such as alumina (Al2O3) is coated on an inner wall surface of a refractory inorganic substrate made of ceramics or metal foil. The refractory inorganic substrate has various shapes as well as a monolithic shape. Such noble metal-supported powder coated on the refractory inorganic substrate is particles with a mean particle diameter approximately ranging from 1 [μm] to 20 [μm]. Independently of gas diffusibility, the noble metal-supported powder dispersively supports, on a surface thereof and in an inside thereof, the noble metal serving as an activity center to purify a harmful gas component in the exhaust gas.
With regard to the exhaust gas purifying catalyst as described above, there is one in which, in a coating layer (catalyst layer), pores with a pore diameter of 0.1 to 20 [μm] occupy 60% or more of the entire pore volume, and pores with a pore diameter of 10 to 20 [μm] occupy 20% or more of the entire pore volume (Patent Citation 1).    Patent Citation 1: Japanese Patent Unexamined Publication No. 2002-191988